Do a bit less work when preserveNewlines is off

src/compiler/emitter.ts

@@ -4278,11 +4278,13 @@ namespace ts {
                     return rangeIsOnSingleLine(parentNode, currentSourceFile!) ? 0 : 1;
                 }
                 else if (!positionIsSynthesized(parentNode.pos) && !nodeIsSynthesized(firstChild) && firstChild.parent === parentNode) {
-                    let lines = getLinesBetweenPositionAndPrecedingNonWhitespaceCharacter(firstChild.pos, currentSourceFile!, /*includeComments*/ true);
-                    if (lines === 0) {
-                        lines = getLinesBetweenPositionAndPrecedingNonWhitespaceCharacter(firstChild.pos, currentSourceFile!, /*includeComments*/ false);
+                    if (preserveNewlines) {
+                        const lines = getLinesBetweenPositionAndPrecedingNonWhitespaceCharacter(firstChild.pos, currentSourceFile!, /*includeComments*/ true);
+                        return lines === 0
+                            ? getLinesBetweenPositionAndPrecedingNonWhitespaceCharacter(firstChild.pos, currentSourceFile!, /*includeComments*/ false)
+                            : lines;
                     }
-                    return printerOptions.preserveNewlines ? lines : Math.min(lines, 1);
+                    return rangeStartPositionsAreOnSameLine(parentNode, firstChild, currentSourceFile!) ? 0 : 1;
                 }
                 else if (synthesizedNodeStartsOnNewLine(firstChild, format)) {
                     return 1;
@@ -4297,8 +4299,8 @@ namespace ts {
                     return 0;
                 }
                 else if (!nodeIsSynthesized(previousNode) && !nodeIsSynthesized(nextNode) && previousNode.parent === nextNode.parent) {
-                    const lines = getEffectiveLinesBetweenRanges(previousNode, nextNode, getLinesBetweenRangeEndAndRangeStart);
-                    return printerOptions.preserveNewlines ? lines : Math.min(lines, 1);
+                    const lines = getEffectiveLinesBetweenRanges(previousNode, nextNode);
+                    return preserveNewlines ? lines : Math.min(lines, 1);
                 }
                 else if (synthesizedNodeStartsOnNewLine(previousNode, format) || synthesizedNodeStartsOnNewLine(nextNode, format)) {
                     return 1;
@@ -4322,7 +4324,7 @@ namespace ts {
                 }
                 else if (!positionIsSynthesized(parentNode.pos) && !nodeIsSynthesized(lastChild) && lastChild.parent === parentNode) {
                     const lines = getLinesBetweenRangeEndPositions(lastChild, parentNode, currentSourceFile!);
-                    return printerOptions.preserveNewlines ? lines : Math.min(lines, 1);
+                    return preserveNewlines ? lines : Math.min(lines, 1);
                 }
                 else if (synthesizedNodeStartsOnNewLine(lastChild, format)) {
                     return 1;
@@ -4334,26 +4336,28 @@ namespace ts {
             return 0;
         }
 
-        function getEffectiveLinesBetweenRanges(
-            node1: TextRange,
-            node2: TextRange,
-            getLinesBetweenPositions: (range1: TextRange, range2: TextRange, sourceFile: SourceFile, includeComments: boolean) => number
-        ) {
-            // We start by measuring the line difference from parentNode's start to node2's comments start,
-            // so that this is counted as a one line difference, not two:
+        function getEffectiveLinesBetweenRanges(node1: TextRange, node2: TextRange) {
+            // If 'preserveNewlines' is disabled, skip the more accurate check that might require
+            // querying for source position twice.
+            if (!preserveNewlines) {
+                return getLinesBetweenRangeEndAndRangeStart(node1, node2, currentSourceFile!, /*includeComments*/ false);
+            }
+            // We start by measuring the line difference from node1's end to node2's comments start,
+            // so that this is counted as a one-line difference, not two:
             //
             //   node1;
             //   // NODE2 COMMENT
             //   node2;
-            const lines = getLinesBetweenPositions(node1, node2, currentSourceFile!, /*includeComments*/ true);
+            const lines = getLinesBetweenRangeEndAndRangeStart(node1, node2, currentSourceFile!, /*includeComments*/ true);
             if (lines === 0) {
                 // However, if the line difference considering node2's comments was 0, we might have this:
                 //
                 //   node1; // NODE2 COMMENT
                 //   node2;
                 //
-                // in which case we should be ignoring node2's comment.
-                return getLinesBetweenPositions(node1, node2, currentSourceFile!, /*includeComments*/ false);
+                // in which case we should be ignoring node2's comment, so this too is counted as
+                // a one-line difference, not zero.
+                return getLinesBetweenRangeEndAndRangeStart(node1, node2, currentSourceFile!, /*includeComments*/ false);
             }
             return lines;
         }
@@ -4386,7 +4390,7 @@ namespace ts {
             }
 
             if (!nodeIsSynthesized(parent) && !nodeIsSynthesized(node1) && !nodeIsSynthesized(node2)) {
-                const lines = getEffectiveLinesBetweenRanges(node1, node2, getLinesBetweenRangeEndAndRangeStart);
+                const lines = getEffectiveLinesBetweenRanges(node1, node2);
                 return preserveNewlines ? lines : Math.min(lines, 1);
             }
 

src/compiler/utilities.ts

@@ -4703,7 +4703,7 @@ namespace ts {
 
     export function getLinesBetweenPositionAndPrecedingNonWhitespaceCharacter(pos: number, sourceFile: SourceFile, includeComments?: boolean) {
         const startPos = skipTrivia(sourceFile.text, pos, /*stopAfterLineBreak*/ false, includeComments);
-        const prevPos = getPreviousNonWhitespacePosition(pos, sourceFile);
+        const prevPos = getPreviousNonWhitespacePosition(startPos, sourceFile);
         return getLineOfLocalPosition(sourceFile, startPos) - getLineOfLocalPosition(sourceFile, prevPos || 0);
     }